1. Field of the Invention
The invention relates to an electronic stethoscope of the type comprising a vibration transducer, an amplifier, and a headphone arrangement.
2. Description of Related Art
Stethoscopes are used by physicians to listen to sounds from the organism, in particular heart and lungs. The phenomena listened for emit sounds with frequencies from below 16 Hz to about 8 kHz, but a serious low pass filtering occurs during the passage of tissue and skin. The skin acts like a transmitter of those signals which are subsequently accessible. The construction of the stethoscope ascertains that only a small area of the skin is listened to at a time, and that sounds in the room outside are dampened, and thus, the signal-to-noise ratio is somewhat improved. Physicians train actively in the use of stethoscopes, and thereby their ability to distinguish signals in the surrounding noise may rise by about 15 dB. This occurs the world over, and one might say that a stethoscope is a universal tool. However, its value and/or performance to the users have fallen, due to the technical development of society. The increased machine noise, in particular in hospitals, in practice, causes the signals to lie below, or at the most at, the lower limit of human hearing. To this may be added that more and more young persons suffer from hearing loss at the time they may embark on a medical education, and hence the acoustic stethoscope has in practice reached its limit of performance.
It has long been realized that a traditional acoustic stethoscope introduces many linear distortions in its signal transmission, in particular because of the possibility for standing waves in the long tubes. This may be expressed differently by stating that the bad impulse response causes a strong distortion of the temporal reproduction of the signals. Traditionally, there are various constructions of stethoscopes, and they each have their individual characteristic transfer function, and one may to a certain degree, by changing acoustical stethoscope, obtain a more distinct representation of a given acoustic phenomenon. However, the physician's reliability in using stethoscopes is generally so large, in particular when using the stethoscope they have become used to, that the impulse response problem has not been regarded as a bar to the use of acoustic stethoscopes.
For this reason, it has not been attractive to use an electronic stethoscope, even though it gives the possibility of active amplification to any desired degree. Even though there are furthermore very improved possibilities for adapting the sensitivity of a vibration transducer to the body being measured upon, and even though headphones with a high damping may act as better insulators against surrounding noise than ordinary earpieces of a traditional stethoscope, an electronic stethoscope still meets resistance, in particular because it does not “sound like what they used to,” due to the wider frequency band and the consequent larger content of noise. The ability to distinguish phenomena which has laboriously been learnt by the physician does not help any longer.
It has been recognized that there is a need for amplifying certain frequency areas relative to those which are effectively reproduced by an acoustic stethoscope. A known construction of an electronic stethoscope is described in U.S. Pat. No. 5,003,605 which electronically performs a lifting of these wider frequency areas and performs a frequency conversion for very low frequency areas. In this way certain phenomena are given an improved clarity. Furthermore, the stethoscope is connected to electrodes and electrocardiographic circuits for the recognition of the QRS complex in order that signals representing the electrical activity of the heart may be brought to the ear simultaneously with the heart sounds so that the temporal relationship of the sounds in relation to the heart cycle may be evaluated.
Other example of an electronic stethoscope can be found in U.S. Pat. No. 4,598,417. In the stethoscope of this patent, a signal processing approach is used which relies upon an acousto-electronic feedback to provide an error or adjustment signal to amplifier gain control circuits. However, such a stethoscope is complex, prone to failure, and expensive to produce. For example, the feedback signal processing approach requires a second microphone and associated electronics for monitoring the sound output of the electronic stethoscope. Furthermore, the feedback microphone wold be prone to performance degradation (e.g., change in frequency response, sensitivity, etc.) over time from either natural aging, exposure to harsh environments and/or abuse by the user. Such performance degradation of the microphone would, in turn, degrade the performance of the device.
Crude attempts have been made to provide a digital stethoscope in which the sound is presented to two ears in a binaural fashion creating a synthetic listening space in which the audible phenomena of interest are distributed spectrally from left to right in order that lower frequencies appear to emanate from one side and higher frequencies from the other side via analog means. U.S. Pat. No. 4,594,731 describes the generation of higher and more audible frequencies from the actual cardiac sounds by means of frequency multipliers, and these sounds are then subjected to an artificial shift left-to-right in the stereo image by means of reciprocal ramp-shaped control signals. It appears that the results obtained are inextricably linked to the simultaneous use of these two principles. However, a physician desiring to use this type of apparatus will first have to learn to listen for completely new sounds and will then have to accommodate the stereophonic signal which for each cycle of synthetic cardiac sounds is similar to turning the balance control on a stereo amplifier from left to right. In U.S. Pat. No. 4,783,814 use is made of a variable time delay to provide signals to left and right ears which have a time delay between them, in order to enable the physician to distinguish time intervals which are less than 40 ms apart. The sounds gain a distinct quality in that the physician perceives a spatial orientation to the sound that is as if the heart sounds come from within his own head. However, neither of these known attempts provide realism to the heart sound, and the sounds produced are so different from the sounds that physicians have been trained to recognize that improved signal processing is necessary to make realistic use of binaural ausculation.
It has also been known since around 1994 that the brain is able to process sounds more effectively when it receives slightly different input from each of the two ears. This fact has been used to improve audio reproduction of music and for films, i.e., stereo and surround sound vs. monaural sound. This ability has also been used by jet airplane pilots to increase separation when listening to several communication channels simultaneously. The use of special fitters for these purposes is known and research has been conducted to determine the filter effect of the head (characterized as Head related Transfer Functions, HRTF=s) with regard to sound from two microphones based on the distance and source; see, HRTF Measurements of a KEMAR Dummy-Head Microphone, Bill Gardner and Keith Martin, MIT Media Lab Perceptual Computing—Technical Report #280, May, 1984, which can be found on the website of the Massachusetts Institute of Technology. However, to date, no known attempt has been made to use this ability in an electronic stethoscope to separate or transform defined sound components so as to present different features to each of the physician's ears.